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BUREAU OF MINES 
INFORMATION CIRCULAR/1989 

J<0 




Practical Considerations in Longwall 
Face and Gate Road Support 
Selection and Utilization 



By Thomas M. Barczak, David E. Schwemmer, 
and Carol L. Tasillo 



UNITED STATES DEPARTMENT OF THE INTERIOR 



Information Circular 9217 



Practical Considerations in Longwall 
Face and Gate Road Support 
Selection and Utilization 



By Thomas M. Barczak, David E. Schwemmer, 
and Carol L. Tasillo 



UNITED STATES DEPARTMENT OF THE INTERIOR 
Manuel J. Lujan, Jr., Secretary 

BUREAU OF MINES 
T S Ary, Director 



TVla5)s 
no.12J7 



Library of Congress Cataloging in Publication Data: 



Barczak, Thomas M. 

Practical considerations in Iongwall face and gate road support selection and 
utilization. 

(Bureau of Mines information circular; 9217) 

Bibliography: p. 22. 

Supt. of Docs. 4io.: I 28.27:9217. 

1. Mine roof control. 2. Longwall mining— Safety measures. I. Schwemmer, 
David E. II. Tasillo, Carol L. III. Title. IV. Series: Information circular (United 
States. Bureau of Mines); 9217. 



TN295.U4 



[TN288] 



622 s [622'.334] 



88-600440 



CONTENTS 



Page 



Abstract 1 

Introduction 2 

Research perspective 2 

Development of practical considerations 4 

Shield support considerations 5 

Support selection and design considerations 5 

Shield type 5 

Component constructions 5 

Support capacity 8 

Performance acceptance testing 8 

Active versus passive load application 9 

Static versus dynamic testing 10 

Critical load evaluations 10 

Operational considerations 12 

In situ performance monitoring 13 

Crib support considerations 14 

Support selection and application 14 

Construction 16 

Failure observations 18 

Laboratory testing and analysis 19 

Conclusions 21 

Bibliography 22 



ILLUSTRATIONS 

1. Mine roof support optimization 3 

2. Organization scheme for shield configurations 4 

3. Organization scheme for gate road supports 5 

4. Location of support resultant resistance 6 

5. Comparison of solid base and split base shield designs 7 

6. Split caving shield design 7 

7. 2:1 canopy ratio 7 

8. Total load determinations on shield supports 8 

9. Shield leg mechanics considerations 9 

10. Friction effects on performance testing of shield supports 10 

11. Most critical contact configuration for two-leg shield supports 10 

12. Critical load conditions for shield supports 11 

13. Direction of displacement loading for shield performance tests 11 

14. Friction-free shield tests 11 

15. Horizontal shield constraint 12 

16. Shield-setting pressure considerations 12 

17. Setting shield with tip raised 13 

18. Advancing support under partial contact with roof strata 13 

19. Measurement of shield loading 14 

20. Wood and concrete crib strength and stiffness 15 

21. Wood crib subjected to horizontal displacement 15 

22. Wood crib stiffness as a function of crib height 16 

23. Construction of wood cribs with overhanging ends 16 

24. Unstable parallelogram wood crib configuration 16 

25. Improving wood crib strength by adding additional blocks per layer 17 



ILLUSTRATIONS-Continued 



Page 



26. Instability in wood crib resulting from weak crib member 17 

27. Composite wood-concrete crib structures 18 

28. Load transfer disks used in concrete crib constructions 18 

29. Typical failure in concrete crib support 19 

30. Explosive failure of concrete crib support constructed with load transfer disks 20 

31. Convergence rate effect on wood crib support resistance 20 

32. End effect considerations in crib testing 20 



UNIT OF MEASURE ABBREVIATIONS USED IN THIS REPORT 


ft 


foot kip/in 2 


kip per square inch 


in 


inch pet 


percent 


in/min 


inch per minute psi 


pounds per square inch 


kip/in 


kip per inch 





PRACTICAL CONSIDERATIONS IN LONGWALL FACE AND 
GATE ROAD SUPPORT SELECTION AND UTILIZATION 



By Thomas M. Barczak, 1 David E. Schwemmer, 2 and Carol L. Tasillo 3 



ABSTRACT 

The U.S. Bureau of Mines has been conducting research to optimize the design and utilization of mine 
roof support systems. An objective of these efforts is to evaluate the mechanical and structural responses 
of various mine roof support systems under simulated load conditions in the Bureau's mine roof 
simulator. Underground studies are also made to evaluate the in situ behavior of support structures. 
The purpose of this report is to document practical applications for longwall face and gate road supports 
that have resulted from these studies. This report is not intended to be an all-inclusive manual on every 
aspect of support utilization, but it does provide a comprehensive assessment of practical considerations 
relating to the mechanical and structural behavior of these support systems. Forty-six recommendations 
are made to provide assistance to mine operators in the testing, selection, and utilization of longwall face 
and gate road supports. Many of these recommendations offer innovative solutions to everyday problems 
faced by mining personnel in the control of ground in longwall mining. 



'Research physicist, Pittsburgh Research Center, U.S. Bureau of Mines, Pittsburgh, PA. 
Structural engineer, Boeing Services International, Pittsburgh, PA. 
3 Senior project engineer, Boeing Services International. 



INTRODUCTION 



The Bureau of Mines has been conducting research to 
optimize the design and utilization of underground mine 
roof support systems. Much of this research was con- 
ducted in the Bureau's mine roof simulator and was in- 
tended to develop fundamental relationships concerning 
the mechanics and structural responses of various mine 
roof support systems under simulated load conditions. 
Several publications are available that extensively docu- 
ment these studies (see "Bibliography" section). These 
publications describe the parameters under consideration 
and test results for each area of investigation. Each report 
contributes to understanding the engineering mechanics 
involved in roof support systems. The reports contain 
detailed theoretical and analytical analyses of support be- 
havior and do not concentrate on practical applications. 
The purpose of this report is to document the practical 
applications that have resulted from these various studies. 

The scope of this report is limited to longwall face and 
gate road supports, specifically shields and various forms 
of wood and concrete cribbing. References are made to 
pillar considerations, but this report is concerned primarily 
with artificial supports. It is not intended to be an all- 
inclusive manual on every aspect of support utilization, but 



represents a fairly comprehensive assessment of the prac- 
tical considerations relating to mechanical and structural 
behavior of these support systems. Concise recommenda- 
tions are made for testing, selection, application, construc- 
tion, and operation of these supports. 

Proper ground control is essential to the safety of 
underground coal mining. Significant improvements have 
been made in longwall ground control during the past de- 
cade, but ground control problems still persist and remain 
a limiting factor in longwall mining productivity. Many 
articles and a few books 4 have been written to assist coal 
operators in ground control techniques. Much is to be 
learned from these publications and from the experience 
gained from mining personnel as they labor to achieve 
success. This report is not intended to be a reiteration of 
this literature or a synopsis of operator experiences. 
Rather, it is intended to advance the state-of-the-art of 
longwall ground control by contributing new information 
as an outgrowth of the Bureau's research in support be- 
havior and strata interaction studies. However, to main- 
tain a comprehensive discussion on the subject of longwall 
face and gate road support considerations, some informa- 
tion of common knowledge is included. 



RESEARCH PERSPECTIVE 



Before discussing practical considerations, a brief over- 
view of the Bureau's research program is presented to pro- 
vide some background for the practical considerations de- 
veloped in this report. Figure 1 shows the overall technical 
approach pursued in the mine roof support optimization 
research. This research combines studies of support 
mechanics with rock mechanics investigations to ensure 
optimum compatibility when considering the interaction of 
the support structure with the surrounding strata in re- 
sponse to various loading conditions. As the diagram in 
figure 1 implies, ground control considerations must in- 
volve evaluations of both the support structure and the 
geological environment. 

The research to date has primarily been in the area of 
support mechanics, using the Bureau's mine roof simulator 
to simulate controlled loading of wood and concrete crib- 
bing and longwall shields. Areas of evaluation for shield 
supports included stability, stiffness characteristics, load 
transfer mechanics, and critical load considerations. 
Parameters under investigation were type of shield (two- 
leg versus four-leg), height, setting pressure, contact 



configuration, direction of applied displacement (vertical 
versus horizontal), and horizontal constraint. Areas of 
evaluation for gate road crib support structures included 
stability, load-displacement relationships, failure mechan- 
ics, material considerations, and geometric considerations. 
Study parameters for wood cribs included load rate, con- 
figuration (contact area), height, wood type, moisture con- 
tent, and horizontal displacement. Compressive strength, 
configuration (solid versus open), square versus round ge- 
ometry, and height were evaluated for concrete cribs. The 
type, amount, and location of wood in composite wood- 
concrete structures was also investigated in these studies. 
This research represents a 5-year effort initiated in 
1983. Future research will investigate other parameters 
that affect support response, such as gob loading on shield 
supports and interaction effects in multiple crib support 
utilization. In addition, technologies developed in the lab- 
oratory will be employed and evaluated underground. 

4 Peng, S. S. Coal Mine Ground Control. Wiley, 1978, 450 pp. 
Peng, S. S., and H. S. Chiang. Longwall Mining. Wiley, 1984, 708 pp. 



I 



Support 
mechanics 



I 



I I 

I Rock | 

i mechanics \ 

&■ j 



• Load transfer 

• Stability 

• Structural integrity 



• Weight of 
unstable strata 

• Convergence 



!• 1 

Interaction | 
| support and strata g 



Vtet*z^.4S^^a*,te*^^t^ 



Load conditions 
Support response 



Support 
selection 



^r^WI^WW»«tff.lWW4 l J'WI^ 



Optimization 



AiVttiaXAMSiiiV&XfwAKftili 



'M&KWW&teWW T WMmq 



Optimum 



compatibility I 



Support 
design 



Maximize \ 

efficiency | 



5 Engineer support to geological conditions J 



Figure 1.— Mine roof support optimization. 



DEVELOPMENT OF PRACTICAL CONSIDERATIONS 



From the research perspective presented in the previous 
section, it is seen that there are several parameters in- 
volved in mechanical and structural analyses of roof sup- 
port behavior. To relate these to practical considerations 
of ground control in longwall mining, a scheme is devel- 
oped to organize this information in specific areas of prac- 
tical interest. Figure 2 depicts an organization scheme for 
shield considerations and figure 3 reveals a similar organ- 
ization for gate road crib supports. 

Practical considerations for shield supports are 
discussed in terms of (1) support selection and design con- 
siderations, (2) performance acceptance testing, (3) opera- 
tional considerations, and (4) in situ performance monitor- 
ing. Topics of discussion for each of these areas are 
shown in figure 2 and include such things as type of shield, 
component constructions, setting pressure recom- 
mendations, methods of testing, and failure assessments. 
Because crib supports are passive supports, operational 



considerations are not an issue. However, as passive sup- 
ports, cribs are less adaptive than shields to changing load 
conditions, and failure is more an area of concern for crib 
supports. Hence, selection of the proper crib support for 
specific conditions is critical. Practical considerations for 
crib supports are discussed in terms of (1) support selec- 
tion and application, (2) construction, (3) failure observa- 
tions, and (4) laboratory testing and analysis. 

The format for presenting practical considerations is to 
identify concise and definite recommendations for applica- 
tion or evaluation of longwall face and gate road support 
systems in the control of ground in longwall mining. 
Accompanying most recommendations is an illustration to 
graphically describe the issue under consideration. Follow- 
ing each recommendation is a brief technical explanation 
to support the recommendation. The 46 recommendations 
are numbered consecutively through the report. 



Shield support consideration 











































Sele< 

ar 

des 


;tion 

id 

ign 




Perfor 
test 


mance 
ing 




Oper 


ation 




In situ monitoring 


• Type of 


shield 




• Active 


versus 




• Setting 


pressure 




• Shield loading 






passive testing 










• Component 




• Static versus 




• Setting procedures 




• Structural failures 


constructions 




dynamic testing 










• Support capacity 




• Critical load 
evaluations 




• Advance procedures 




• Analysis methods 












• Yield pressure 







Figure 2.— Organization scheme for shield configurations. 



Crib support consideration 











































Selection 

and 




Construction 




Failure 




Laboratory 


applic 


ation 






1 — -J 




observations 




tesl 


Mng 


• Concrete 
wood 


versus 
crib 




• Concrete 
wood 


' versus 




• Expected failure 
modes 




• Size effects 


• Strata conditions 




• Geometries 




• Signs of failure 




• Rate effects 


• Seam height 




• Stiffness 




• Potential danger 




• Machine stiffness 


• Convergence 




• Strength 










• Sustained loading 



Figure 3.— Organization scheme for gate road supports. 



SHIELD SUPPORT CONSIDERATIONS 



SUPPORT SELECTION AND 
DESIGN CONSIDERATIONS 

Topics of discussion in support selection and design 
considerations are shield type, component constructions, 
and support capacity considerations. This section provides 
information on when to use what support, how to evaluate 
required support capacity, and what to consider and avoid 
in specific shield designs. 

Shield Type 

1. Use four-leg shield designs in conditions where can- 
tilevered strata is likely and two-leg shields in less com- 
petent strata conditions (fig. 4). 

Four-leg shields have a larger capacity potential than 
two-leg shields because of the larger number of leg cylin- 
ders, and are therefore better able to support larger loads 
caused by cantilevered strata. Four-leg shields also pro- 
vide a resultant resistance force closer to the gob than 
two-leg shields. This reduces required support capacity to 
maintain moment equilibrium, improves support stability, 
and may help induce breakage of the strata to alleviate the 



cantilevered strata condition. In contrast, two-leg shields 
provide a resultant resistance force closer to the face, and 
they are more effective in controlling less stable strata 
prone to cavity formation in the face area. In addition, 
two-leg shields are likely to have a shorter canopy length 
and produce less cycling of roof strata. 

Component Constructions 

2. Use solid base configurations only in soft bottom con- 
ditions where bearing pressures of split base designs 
exceed the strength of the floor strata (fig. 5). 

Structurally, solid bases are more stable because they 
provide an out-of-plane stiffness. The larger contact area 
provided by the solid design provides a lower bearing 
pressure on floor strata, making solid base designs more 
suitable in soft bottom conditions. Despite these structural 
advantages, the trend has been toward split base configura- 
tions. Split base configurations more easily accommodate 
floor irregularities during support advancement and have 
sufficient flexural stiffness to adequately distribute loads 
for most floor strata. 



LongwallWIj 



face 







^mm^m^w^iw^^^^^^^m^^T^^m^ 



Support response 



1 



Long wall tM 



face 



Resultant 
force / 'F^ 



Four- leg 
shield 



; #\w?^ 



#W^J^AW^^^^ 




Figure 4.— Location of support resultant resistance. 



3. Avoid split caving shield designs (fig. 6). 

Split caving shields are utilized to permit out-of-plane 
rotation of the canopy to enable the canopy to better 
maintain contact with irregular roof strata. Split caving 
shield designs reduce out-of-plane shield stiffness, and the 
shield is less able to resist out-of-plane (lateral) loading. 
The result of this design is often an unstable configuration. 
Nearly all installations using the split caving shield design 
have experienced stability problems. 

4. Solid canopy designs provide the most effective roof 
control. 

Articulated canopy designs are becoming obsolete. 
Canopy articulation generally reduces shield stability. 
Flipper arrangements at canopy ends prevent debris from 
falling but probably do little to control roof behavior or 
inhibit failure. Loads generated in canopy extensions must 
be transmitted through the canopy structure, and therefore 
add to the strength requirements of the canopy design. 
They should be avoided except in very friable roof 



conditions where added protection is required in the un- 
supported span between the coal face and the canopy tip. 

5. The requirement of a 2:1 canopy ratio is only a rule of 
thumb and should not be a primary design consideration 
(fig- 7). 

The 2:1 canopy ratio relates the location of the resultant 
support resistance from the canopy tip to the distance 
from the resultant force to the canopy-caving shield hinge. 
It is intended to ensure an adequate active force over the 
full length of the canopy, particularly at the canopy tip. 
The effectiveness of the canopy to maintain adequate roof 
pressure along its full length is significantly dependent 
upon the flexural stiffness of the canopy structure. As the 
flexural stiffness decreases, less tip loading is likely. 
Another significant consideration that violates the 2:1 re- 
quirement is the shape of the canopy. Some canopies are 
shaped concavely upward to ensure tip loading. Canopy 
structures that promote initial contact at the tip should not 
be steadfastly evaluated by the 2:1 rule. 




Figure 5.-Comparison of solid base and split base shield designs. 




Split 
caving shield 



Canopy ratio = jj- 



•H-«— fa- 




Figure 6.— Split caving shield design. 



Figure 7.-2:1 canopy ratio. 



6. Be cautious of lemniscate link designs that promote 
bending of the link structure. 

Some two-leg shield supports use "curved" link designs 
where the line of action between the pin holes is not con- 
sistent with the centroid of the member. This is usually 
done to provide adequate clearance at collapsed heights. 
These designs impose eccentric loading causing potentially 
large stresses due to bending, which must be considered in 
the design. One support tested in the simulator reached 
material yield in the link at much less than rated support 
capacity. With the exception of pin friction, links need 
only be designed for axial loads. 

Support Capacity 

7. Support capacity determinations should consider support 
stiffness and face convergence (fig. 8). 

Historically, the most common analytical method to 
determine support capacities has been to evaluate caving 
characteristics using some form of bulking factor formula- 
tion to estimate a rock mass that must be maintained in 
equilibrium by the support resistance. Estimation of cav- 
ing characteristics is difficult and is generally not an accu- 
rate means to assess support capacity. Support resistance 



required to maintain equilibrium of the rock mass as eval- 
uated in these or other methods should be used to assess 
required setting forces and not yield loads. The total load 
on the support will be the sum of the setting force and the 
reactive load due to convergence, which is a function of 
the support stiffness. Hence, support capacity determina- 
tions should be based upon face convergence and support 
stiffness. Hsiung proposes a methodology using equivalent 
support-strata stiffness and face convergence to size shield 
supports. 5 The Bureau is currently developing a similar 
methodology using shield stiffness and face convergence 
parameter considerations. 

PERFORMANCE ACCEPTANCE TESTING 

This section provides information on how to test shields 
and evaluate their performance in the laboratory. Topics 
for discussion include issues of active versus passive 
testing, static versus dynamic testing, and critical load 
evaluations. 



5 Hsiung, S. M., Y. M. Jiang, and S. S. Peng. Method of Selecting 
Suitable Types of Shield Supports at Longwall Faces. Paper in Pro- 
ceedings of the Seventh International Conference on Ground Control 
in Mining. WV Univ. 1988, pp. 161-168. 




Setting load (S) = W 




Convergence (S) 



K = Shield stiffness 



Convergence load (R)= K-*8 



Total load (F)= Setting load (S)+ convergence load (R) 

F= W + K*8 



Figure 8.— Total load determinations on shield supports. 



Active Versus Passive Load Application 

8. Performance testing on shields with double telescoping 
leg cylinders by passive load application, where leg pres- 
sures are used to generate shield loading by reactions 
against a static frame, should be conducted at less than 
full first stage leg extensions to be compatible with in 
mine behavior and performance testing by active load 
application (fig. 9). 

Because of leg mechanics, the effective area of the leg 
cylinder changes for leg convergence in comparison to leg 
extension when the first stage is fully extended. Tests con- 
ducted in the Bureau's mine roof simulator indicate that 
leg force is reduced by 50 pet on some shields when the 
first stage is fully extended and pressurized to produce leg 
extension. This reduction in leg force will provide lower 
stresses in the support structure, and will result in errone- 
ous evaluations of the support's structural integrity. This 
in turn may jeopardize the safety of the miners in the 
event that critical loading was ignored by not providing the 
maximum leg force. 



9. Leg pressures should be higher than in-service yield 
pressure for passive load application (testing in static 
frames) to achieve equivalent results at yield pressure 
under active load application (testing in active frames) 
(fig. 10). 

When the shield is developing reactions to applied dis- 
placements that cause compression of the leg cylinders, 
the friction in the leg cylinder and the caving shield-lem- 
niscate joints acts to help resist the applied displacement 
and therefore increases support capacity. Conversely, in a 
static frame where the leg cylinders are pressurized there- 
by causing leg extensions, the friction opposes the leg 
forces and caving shield-lemniscate assembly stiffness caus- 
ing a reduction in support resistance. Friction effects are 
likely to be on the order of 3 to 5 pet of the total support 
resistance, but can reach 10 pet under certain load condi- 
tions. The friction effects can be evaluated by examination 
of the hysteresis in load-unload responses during testing. 



LEG EXTENSION 

F 



LEG CONVERGENCE 



TTTT 
0|A 2 



Leg force (F) =O t A 2 




A, 

A 2 
0~|. 2 



KEY 
Leg force 
Trapped fluid 
Pressure application 
Area of 1st stage 
Area of 2nd stage 
Cylinder pressure 



Leg force (F)=0| A t 

Figure 9.— Shield leg mechanics considerations. 



10 



Static frame testing 




U I ) ) I I I / / / A 

KEY 

»• Leg force (L) 

" Frictional force (f) 

Active load application ^ — 1> Support resistance (F) 



f 



<h> l / I I / I / / / ) / I / / l_ 3 <| 



F =L+ f 




Figure 10.— Friction effects on performance testing of shield 
supports. 




Figure 11— Most critical contact configuration for two-leg 
shield supports. 



minimize costs. This practice is acceptable, but tests 
should be prioritized with the most critical load conditions 
evaluated last. Otherwise, the worst load case may pro- 
duce component stresses that damage weaker components 
and mask the effect of less critical load conditions. Test 
results at the completion of each test series should be 
analyzed so that effects of each load case are better 
understood. 



Static Versus Dynamic Testing 

10. Static tests should be used to evaluate strength of mate- 
rial considerations in component designs and cyclic tests 
used to evaluate material fatigue and weld integrity. 

All support structures should first be analyzed under 
static load conditions to eliminate any load rate effects and 
to provide an understanding of load transfer within the 
structure. Fatigue studies are also an important considera- 
tion and should be conducted under all load conditions 
representative of underground conditions. Cycling should 
be done over the full range of support loads, ideally from 
zero to yield load. Conditions that produce a change in 
stress from tension to compression or vice versa are gen- 
erally more critical than conditions that maintain the same 
stress state. Cycle rate may also have a significant influ- 
ence on fatigue failure. Generally, lower frequency load 
application (load rate per cycle) is more critical than 
higher frequency load application. 

11. Cumulative cycle tests should be prioritized and the 
most critical load condition conducted last. 

Generally, all tests are conducted on one support to 
observe cumulative effects of all load conditions and to 



Critical Load Evaluations 

12. The most critical contact configuration for two-leg shield 
supports is standing the support on the toe of the base 
(fig- U). 

Simply supporting the base on its toe with any canopy 
contact configuration causes maximum loading in the base 
member and lemniscate links. In this configuration the 
base is subjected to maximum bending moment as the rear 
link acts in tension to pull the rear of the base upward 
while the leg force pushes it down. The lemniscate links 
also are subjected to maximum loading because they must 
act to offset leg forces to provide equilibrium of the base 
to maintain the base-on-toe configuration. 

13. Specific canopy and base contact configurations should 
be designed to evaluate critical loading in each compo- 
nent of the support structure (fig. 12). 

A critical contact configuration for one component may 
not be critical for another component. Critical canopy 
contact configurations are largely independent of base 
contact and critical base contact configurations are in- 
sensitive to canopy contacts. Contact configurations that 
should be considered in critical load testing of shield sup- 
ports are illustrated in figure 12. 



11 



SYMMETRIC CONTACT CONFIGURATIONS 






V 

Full contact 




Canopy bending 



"tcdlcj 



Base bending 






Base-on-toe 



Base -on-rear 



Leg socket 



11/ 



KEY 

X Critical load member 
V Contact 



UNSYMMETRIC CONTACT CONFIGURATIONS 



Canopy 


^3 


up 


^3 fc£S 


+ + 




+ + 


SS3 ^ 




K 



+ 




+ 



Base 

Toe^ 



+ 



KEY 
ESS Contact 
+ Leg connection 



14. 



Figure 12.— Critical load conditions for shield supports. 



Performance testing of shield supports should be con- 
ducted by applying or inducing both vertical and hori- 
zontal displacements to the support structure (fig. 13). 



The caving shield-lemniscate assembly has very little 
vertical stiffness (0-30 kips/in) and will not be significantly 
loaded by vertical displacements (roof-to-floor conver- 
gence). Horizontally, the caving shield-lemniscate assem- 
bly is quite stiff (400-900 kips/in), and large loads can be 
generated, provided horizontal freedom in the pin joints is 
overcome by the horizontal displacement or from horizon- 
tal constrainment during setting. Vertical displacements 
are likely to produce larger stresses in the canopy and 
base, since bending is the primary loading mechanism for 
these components. Horizontal displacements will add to 
the bending of these members. 

15. Friction-free or zero horizontal load tests should be 
conducted to evaluate two-leg shield stability and link 
loading (fig. 14). 

When sufficient friction exists between the canopy and 
base strata interfaces, frictional forces are developed which 



VERTICAL 
DISPLACEMENT 




FACE- TO -WASTE 
HORIZONTAL DISPLACEMENT 




WASTE -TO -FACE 
HORIZONTAL DISPLACEMENT 




Figure 1 3— Direction of displacement loading for shield 
performance tests. 



/ /_/_/ / /_/_ Z I 'CL t ~ r Z T . 



■■■ •» 



• ♦• •• 



KEY 
— ^ Resultant force 

O Restraint 
#"•"• Roller 
— c=» Shield displacement 




Figure 14.— Friction-free shield tests. 



oppose the horizontal component of the leg forces. This 
directs the line of action of the applied resultant force 
away from the toe of the base, thereby aiding in support 
stability and reducing peak pressure distribution on the 
support base in the toe area. The frictional forces also 
minimize link loading because horizontal displacement of 
the canopy-caving shield joint is minimized. In the 
absence of these frictional forces, stability must be pro- 
vided by the caving shield-lemniscate assembly. Therefore, 



12 



friction-free tests are considered to be a critical load con- 
dition for two-leg shield supports. These tests can be con- 
ducted by allowing the load applying platens of a biaxial 
active test frame to displace freely in the horizontal direc- 
tion or by placing rollers or bars on top of the shield can- 
opy or under the base in a static frame test. 

16. Shields should be horizontally constrained during set- 
ting and load application to remove freedom in the pin 
joints during critical load testing (fig. 15). 

Tests conducted in the Bureau's simulator revealed that 
considerable freedom exists in the numerous joints of a 
shield structure, and that loads will not be developed in the 
caving shield-lemniscate assembly unless the shield is prop- 
erly constrained to remove this freedom. Some contact 
configurations, such as base-on-toe contact, require con- 
strainment to maintain stability of the configuration, while 
others, such as full canopy and base contact, can be main- 
tained without constrainment. It is recommended that the 
canopy be displaced horizontally to remove pin freedom 
prior to the test and that link activity be monitored as an 
indication of load generation in the caving shield-lem- 
niscate assembly during critical load testing of shield 
supports. 

OPERATIONAL CONSIDERATIONS 

17. Use higher setting pressures for high shield heights where 
the lower stage of the leg cylinder is fully extended 
(fig. 16). 

Shield setting force is provided by the vertical compo- 
nent of the leg force generated by the applied setting pres- 
sure. Despite the more efficient geometry consideration 
at high shield heights, the setting force can be considerably 
smaller than that provided at lower shield heights where 
the leg is operating at more of an inclined angle from the 
plane of the canopy. Reductions in setting force will occur 
when the lower stage of the leg cylinder is fully extended. 
The reason for this reduction in setting force is that the 
effective area of the leg cylinder is smaller when in this 
configuration. At high shield heights, when the first stage 
is fully extended against the mechanical stops of the leg 
cylinder casing, setting force equals the product of the 
applied setting pressure (measured in the lower stage) and 
the leg area of the upper stage. When the lower stage is 
not fully extended, the pressure in the upper cylinder ex- 
ceeds the lower cylinder pressure in proportion to the ratio 
of the areas and results in a larger setting force. Tests on 
several shields in the Bureau's simulator have shown re- 
ductions in setting forces of up to 50 pet when the lower 
stage is fully extended. 



18. Setting pressure should be based upon face convergence 
and shield stiffness (fig. 8). 

Optimum setting pressure is the m inimum pressure that 
provides stability and equilibrium of the strata. The goal 
is not to prevent convergence, but to provide a setting force 
which is compatible with shield stiffness so that the load 
generation from the resulting convergence is consistent 
with the yield load capability of the support. Hsiung 
promotes a similar concept employing the stiffness rela- 
tionship of the support-strata system and measured face 
convergence. 6 Shield stiffness characteristics for various 



^ork cited in footnote 5. 



—4 



A 0.05 F, 



0.95F 



0.98R 




UNCONSTRAINED SHIELD SETTING 



\ 0.55 R, 




KEY 

► F v Vertical force 
^F H Horizontal force 
t-* Constraint 



CONSTRAINED SHIELD SETTING 

Figure 1 5.— Horizontal shield constraint 




Figure 1 6.— Shield-setting pressure considerations. 



13 



setting pressures and shield heights have been evaluated 
by the Bureau under controlled loading in the mine roof 
simulator. Stiffness characteristics should be made avail- 
able by support manufacturers to operators for analysis 
when purchasing supports. 

19. Avoid setting shield with canopy tip raised up (fig. 17). 

All critical load configurations should be avoided if pos- 
sible. Generally, the operator has little control over the 
contact configuration established from setting the support, 
but the operator can control the attitude of the canopy, 
and setting the support with the canopy tip raised can be 
avoided. Setting the support with the tip up usually results 
in the support standing on the toe of the base which, as 
described earlier, is the worst load case for two-leg shield 
supports. Setting the support with the canopy tip raised 
also produces maximum bending in the canopy structure. 

20. Advance the support under partial contact with the roof 
(fig- 18). 

To the extent practically possible, shields should be 
advanced under partial contact with the roof. This will 
ensure setting the shield in a constrained configuration, 
which removes freedom in the pin joints and increases 
support capacity and stability. In a constrained configura- 
tion, the caving shield-lemniscate assembly will develop 
vertical and horizontal reactions that act to increase sup- 
port resistance against both vertical and horizontal dis- 
placements. The constrained configuration will also result 
in a greater setting force for the same leg pressure. 

IN SITU PERFORMANCE MONITORING 

21. Monitor leg pressures to provide an overall indication of 
shield loading but recognize the limitations of leg pres- 
sure measurements in shield load analysis (fig. 19). 

The easiest and best overall indicator of shield loading 
is an assessment of leg pressures. Vertical loading applied 
to the roof support by strata weight or convergence can be 
reasonably estimated from measurement of leg pressures. 
Vertical resistance provided by the caving-shield-lemniscate 
assembly will be a source of error in determination of 
vertical support loading by leg pressure measurements, but 
this error is likely to be less than 5 pet of the total vertical 
force. The area and pressure of the lower stage of the leg 
cylinder should be used to determine leg force. Horizontal 
forces cannot be accurately determined from leg pressure 
measurements. The Bureau has developed several tech- 
niques to measure horizontal loading. Horizontal loading 
should be measured if support stability or structural fail- 
ures are in question. 



22. Inspect welds for signs of fatigue failures. 

Welds should inspected during each face move to assess 
fatigue loading of shield structures. Visual inspection 
should be adequate as a first means of inspection. Ob- 
served or expected problem areas can be treated with dye 
penetrant for further evaluation. The base appears to be 
the member that often fails first, but all members should 
be inspected. Particular attention should be paid to the 
leg socket area of the base member. Leg sockets are fre- 
quently cast members that are more difficult to weld, and 
the leg socket typically represents an area of stress concen- 
tration in most shields. 

23. Be aware in making evaluations of structural problems 
that failed components alter load transferring 
mechanisms and can obscure the cause of the problem. 

Observed failure of one component does not necessarily 
indicate the source of the problem. When supports are in 
service underground, it is often very difficult to physically 
examine all of the support structure. This can result in 
misinterpretation of problem areas. For example, several 
cases have been brought to the Bureau's attention where 

. _ ~^^ __ Roof strata ~ — — -^ 




WW' 



Figure 17.— Setting shield with tip raised. 



R00f — _ 



Friction with roof strata 



^Lg^SLj^^^ w^---^.-^ 




Floor "" 

Figure 18.— Advancing support under partial contact with roof 
strata. 



14 



ELASTIC MODEL 



Measure shield 
displacements 8 V .8 H 

Determine shield stiffness 



F v = K, 8 V + K 



F h= 



K, 8, 



2 °h 
+ K 4 8 h 




INSTRUMENTED HINGE PIN 



Mec 


isure 


leg pressure 


and 


pin forces 


V 


L v + 


p v 


F H = 


L v + 


P H 




P v Instrumented 
t pin 

©-P, 



LINK STRAIN MODEL 



Measure leg pressures 
and frontor rear link strain 




L*V L + 
L*H L + 


F*V F 
F*H F 


v L . 


v F . h l , 


Hp = Geometric 
configuration 
coefficients 



Link 
♦* strain. 

(L) ' 

Leg pressure 




Figure 19.— Measurement of shield loading. 



mine personnel observed structural failure of the caving 
shield near the lemniscate pins, when the real cause of the 
problem was failure of the base structure that was unno- 
ticeable while the support was in service. Mistakes can 
also be made in overstrengthening a component as a re- 
sult of a failure. Overstrengthening of one component 
could alter the load transfer and result in subsequent fail- 
ure of another component. Careful consideration should 
be given in making major modifications to the structural 
design of the support. Ideally any modification should be 
laboratory tested prior to returning the component to field 
service. 

24. Consideration should be given to fracture mechanics of 
shields constructed of high-strength steel. 

Structural problems with shield supports are not likely 
to be catastrophic. Most longwall support structures are 
constructed of relatively mild steel (40-60 kips/in 2 yield 
strength) with good ductility, which allows considerable 
deformation before failure. Some support components, 
canopies in particular, are constructed of high strength 
steel, 100 kips/in 2 yield strength or greater. These steels 
are significantly less ductile and are more likely to fracture 
when loads exceed the elastic design strength. High- 
strength steels are also more difficult to weld and may be 
more susceptible to fatigue failures than components 
constructed of milder steel. Cracks in any steel structure 
represent a potential loss of structural integrity and 
constitute a potential safety hazard, but any crack in a 
high-strength steel construction should be perceived as a 
sign of imminent danger. 



CRIB SUPPORT CONSIDERATIONS 



This section describes wood and concrete crib behaviors 
and their application as gate road supports in longwall 
mining. Topics of discussion include selection criteria, 
construction considerations, and analysis of crib supports 
by controlled testing in the laboratory. 

SUPPORT SELECTION AND APPLICATION 

25. Use concrete cribs where high strength is required and 
wood cribs where concrete stiffness is not compatible 
with expected convergence (fig. 20). 

A goal in crib support selection for any application is to 
design a structure with a stiffness that is compatible with 
the expected convergence. Reinforced concrete cribs are 
stiff structures that normally fail at vertical convergence of 
2 to 3 pet of the height of the crib, whereas wood cribs are 
very flexible structures and are capable of deforming ver- 
tically 35 to 40 pet of their height in response to a converg- 
ing mine roof. Concrete crib stiffness can be reduced to 
provide more yield capability by reducing the strength of 
the concrete or by incorporating wood within the support 



structure. These issues are further discussed in the "Con- 
struction" section. 

26. Adjust crib stiffness and density across entry widths to 
be compatible with convergence profiles. 

Depending on gate road pillar design and abutment 
loading, convergence is likely to vary across the width of a 
longwall entry. Minimum convergence can be expected to 
occur near the panel. Crib stiffness can be modified (by 
the application of wood volume in concrete cribs) to be 
compatible with convergence at the location of the crib 
support. 

27. Do not use steel-fiber-reinforced concrete cribs in appli- 
cations where the shearer is required to cut through the 
cribs. 

Crib supports are used in recovery room operations to 
provide additional support during face moves. Concrete 
supports are sometimes used in place of wood supports 
because they have high strength and the shearer can cut 



15 



through them during the recovery operation. The steel 
fibers in reinforced concrete cribs tend to hold the con- 
crete together as the shearer cuts the crib. Large uncut 
pieces jam the face conveyor and stageloader. Therefore, 
nonreinforced concrete is preferred for these applications. 
Because nonreinforced concrete can fail violently, it is 
suggested that the crib be surrounded by something to 
contain the material during failure. One mine poured 
nonreinforced concrete underground in tubes to form 
small pillars that effectively controlled the ground during 
longwall recovery. 

28. Use concrete cribs at crib heights where wood cribs 
experience buckling failure. 

Concrete is more uniform and hence provides for better 
stability in crib constructions than wood, which has incon- 
sistent material properties. Stable concrete crib structures 
in excess of 30 ft in height (with cross-sectional areas com- 
parable to 6-ft-high wood crib constructions) have been 
reported. 

29. Use wood cribs in areas of high horizontal displacement 
(fig. 21). 

Wood cribs are able to deform sufficiently to remain 
stable against horizontal displacements (displacements 
applied perpendicular to the longitudinal axis of the sup- 
port). Tests conducted in the Bureau's simulator have 
shown wood cribs greater than 6 ft in height can withstand 
12 in of horizontal displacement with 24 in of vertical dis- 
placement and remain stable without significant loss of 
support capacity. This capability decreases as the cribs are 
reduced in height. At shorter heights, the cribs become 
stiffer and are less able to deform. The shortest wood crib 
tested was 50 in. It remained stable but exhibited a 35-pct 
reduction in support resistance (vertical load capacity) at 
12 in of horizontal displacement. 




30. Increase the density of crib supports as the seam height 
increases (fig. 22). 

The stiffness of wood crib structures increases nonlin- 
early as the height of the structure is reduced, resulting in 
larger load reactions per unit convergence. As an example 
of the effect of height on wood crib resistance, a 110-in- 
high square crib had 35 pet less support resistance than a 
50-in-high crib at 10 in of displacement and 60 pet less 
resistance at 20 in of displacement. 




Figure 20.— Wood and concrete crib strength and stiffness. 



Figure 21.— Wood crib subjected to horizontal displacement 



16 



CONSTRUCTION 

31. Wood cribs should be constructed with minimum 6-in 
overhang on the ends of the blocks (beyond contact area 
between block layers) to enhance stability (fig. 23). 

Overhanging the ends of the crib blocks causes the 
blocks to interlock as they deform from convergence, pro- 
viding a rotational restraint between blocks that significant- 
ly improves stability. Weaker crib blocks have a tendency 
to roll as they fail and cause instability in the crib struc- 
ture. The rotational restraint provided by overhanging 
ends helps to prevent this action. Horizontal displace- 
ments further aggravate block rotation and crib instability. 
Overhanging the ends of the blocks is an effective means 
of improving stability against horizontal displacements. 



32. Wood cribs should be constructed in square geometries 
(fig. 24). 

The strength of wood cribs is proportional to the prin- 
cipal moment of inertia of the crib structure. Wood cribs 
that have the same principal minimum moment of inertia 
will have the same load carrying capability. Therefore, 
although a rectangular crib has a larger maximum moment 
of inertia than a square crib equal in width to the rect- 
angular crib, both have the same minimum moment of 
inertia and will have the same load carrying capability. 
Because the square crib consumes less material and space, 
it is the more optimum configuration. 



800 



600 



o 

O 400 






KEY 
Nominal height 

50 in 

60 in 

80 in 

1 10 in 



200 




6 8 10 12 14 16 18 
VERTICAL DISPLACEMENT, in 



22 



Figure 22.— Wood crib stiffness as a function of crib height. 





Figure 23.— Construction of wood cribs with overhanging ends. Figure 24.— Unstable parallelogram wood crib configuration. 



17 



33. Use additional blocks per layer to improve the strength 
of wood crib supports (fig. 25). 

Experiments were conducted by the Bureau to see if 
increasing interblock contact area by changing the con- 
struction of wood cribs from square to parallelogram 
geometries would increase the strength of the wood crib. 
Results showed that only a minimal increase in strength 
was achieved, with a reduction in stability and support 
resistance at large displacements (greater than 10 in). It 
was concluded that square geometries are a better overall 
configuration, and that increases in strength should be 
provided by adding additional blocks to crib layers. 

34. Wood cribs should be constructed from the same type 
of wood (fig. 26). 

A primary failure mechanism for wood crib supports is 
instability, generally caused by differential compression or 
rotation of individual crib blocks caused by variation in 
their material properties. Construction of wood cribs from 
the same material type will minimize localized differences 
in crib deflections and enhance stability. 

35. Incorporate wood in concrete crib constructions to re- 
duce the stiffness and enhance the yield capability of 
the structure (fig. 27). 

If additional strength and yield capability are desired, 
layers of wood should be added between the layers of con- 
crete. The wood layers act to provide a more uniform 
load distribution on the concrete elements and provide 
constraint against lateral expansion. Tests have shown that 
thin sheets of plyscore or plywood added between each 



layer of concrete will increase the strength of the crib by 
a factor of 2 to 3. If additional yield capability without 
additional strength is required, wood additions should be 
concentrated at the top or bottom of the structure. The 
increase in yield and reduced stiffness of these composite 
configurations will be proportional to the amount of wood 
incorporated in the structure. The incorporation of wood 
in concrete cribs to improve strength or yield capability is 
effective for both circular and rectangular crib geometries. 

36. Use lower strength concrete to reduce the strength of 
concrete crib supports. 

If the reactions developed in concrete cribs exceed the 
bearing strength of the roof or floor strata and wood cribs 
are incapable of providing the desired resistance, use a 
lower strength concrete in the concrete crib design. 




"Weak crib 
■ element 




Figure 25.— Improving wood crib strength by adding additional 
blocks per layer. 



Figure 26.— Instability in wood crib resulting from weak crib 
member. 



18 



High-strength concrete (7,000-8,000 psi) is typically used in 
concrete crib construction to achieve maximum strength. 
A specific strength concrete can be used to provide a crib 
that is most compatible with the conditions in which the 
crib is to be employed. 

37. Concrete cribs should be constructed to minimize non- 
uniform loading (fig. 28). 

Effort should be made to ensure as uniform load as 
possible on concrete crib supports. Eccentric loading or 
concentrated loads (point loads) can significantly reduce 
the strength of the crib. Use wood wedges and crib blocks 
to maintain symmetric loading at roof and floor interfaces 
and remove any debris between layers during crib con- 
struction. Ideally, concrete cribs should be constructed on 
stable floor and loose rubble should be removed prior to 
crib construction. Additionally, wood (load transfer disks) 
can be incorporated between layers of blocks to provide 
more uniformly distributed loading on each block. 



FAILURE OBSERVATIONS 

38. Wood cribs provide visual and audible signs of loading 
and rarely fail catastrophically. 

Crackling or popping noises are frequently heard as 
wood cribs take load. Maximum out-of-plane deflection 
will occur in the middle of the wood crib structure (typical 
of column buckling). If large horizontal displacement of 





Figure 27.— Composite wood-concrete crib structures. 



Figure 28— Load transfer disks used in concrete crib 
constructions. 



19 



the roof relative to the floor occurs, the crib profile will 
resemble more of an S-shape. 

39. Watch for individual crib blocks that have rotated rela- 
tive to the cross section of the crib structure (fig. 26). 

If one or more crib blocks rotate out-of-plane to the 
cross section of the crib structure, the crib could become 
unstable and kick out under load. This condition usually 
occurs with a weak or deteriorated crib block, in crib con- 
structions where the ends were not overhung, or in condi- 
tions where the roof is displacing horizontally relative to 
the floor. 

40. If concrete cribs are crushing out, their stiffness is prob- 
ably incompatible with the ground convergence. 

Properly designed concrete cribs should not fail. Con- 
crete has more than adequate strength to support gravity 
loading of unstable strata in nearly all applications. If they 
are failing, it is more than likely from excessive conver- 
gence that is incompatible with the material stiffness. 
Methods to reduce stiffness and improve yield capability 
have already been discussed. 



LABORATORY TESTING AND ANALYSIS 

43. Wood and concrete cribs tested in the laboratory will 
exhibit greater strength than cribs utilized underground if 
the laboratory convergence rate is greater than the under- 
ground convergence rate (fig. 31). 

Both wood and concrete have load rate dependencies 
that affect crib strength. Tests conducted on wood cribs by 
the Bureau indicated a 30-pct increase in wood crib resis- 
tance for convergence rates of 0.1 in/min or greater com- 
pared to a convergence rate of 0.005 in/min. Load rate 
studies have not been made on full-scale concrete cribs, 
but laboratory tests on concrete test cylinders have shown 
similar load rate dependencies. 

44. End effects can significantly affect postyield behavior of 
concrete cribs (fig. 32). 

Frictional forces developed along interface boundaries 
between cribs and test machine platens or the under- 
ground environment affect the distribution of stresses in 
the specimen and are referred to as end effects. Tests on 



41. Concrete cribs do not provide signs of loading until fail- 
ure is imminent (fig. 29). 

Crack initiation in cementitous materials occurs at the 
microscopic level at about 30 pet of ultimate load, but 
macroscopic crack growth is not visually observable until 
95 to 100 pet of the ultimate strength is reached. Many 
times concrete cribs will fail with little or no warning. 
Generally, crack formation will initiate near the middle of 
the crib structure and progress upward and downward. 
Shear type failures are common for solid crib constructions 
from rectangular blocks or circular disks. Open cribs con- 
structed of rectangular blocks will generally fail at inside 
contact boundaries, which are areas of high stress concen- 
trations. The incorporation of wood layers appears to 
change the failure mechanics of concrete cribs. Composite 
wood-concrete cribs show vertical crack propagation, which 
is indicative of tensile failure from uniaxial stress fields. 

42. Composite wood-concrete crib structures have higher 
energy absorption capability and increased potential for 
explosive failure (fig. 30). 

Wood provides for more uniform loading and acts to 
provide a confinement to lateral expansion. This enhances 
the observed strength by allowing the crib to generate 
pressures closer to the unconfined compressive strength of 
the concrete, but is also disadvantageous in terms of 
energy absorption. Concrete cribs dissipate energy by 
crack growth and plastic deformation. More uniform 
loading and constraint to lateral expansion causes more 
strain energy to develop and less crack growth, which can 
result in highly explosive failures. 




Figure 29.— Typical failure in concrete crib support. 



— ^^mtm^^m^ 



20 




Figure 30.— Explosive failure of concrete crib support constructed with load transfer disks. 



300 



250 



- 200 



< 



CO 
UJ 

ai 

99 
ce 




150 



100- 



0.02 004 0O6 0O8 

CONVERGENCE RATE, in/min 



010 



Figure 31.— Convergence rate effect on wood crib support 
resistance. 



Triaxial state 
of stress 



Uniaxial state 
of stress 



Triaxial state 
of stress 




END ZONE 
CENTRAL ZONE 
END ZONE 



Figure 32.— End effect considerations in crib testing. 



21 



small-scale concrete cylinders have shown that specimen 
behavior during loading (prior to reaching ultimate 
strength) is largely independent of end effects, but behav- 
ior after reaching ultimate strength (postyield behavior) is 
significantly dependent on frictional boundary considera- 
tions. Laboratory test results indicate that postyield load 
carrying capability is significantly reduced if the frictional 
restraint is reduced. 



energy that is available, the faster a crack will grow. 
Ideally, a stiff testing machine is desirable so that energy 
is not built up in the test frame and released to the speci- 
men during testing, as this would accelerate crack growth 
and reduce crib strength. 

46. Sustained loading will reduce the strength of concrete 
cribs. 



45. Stiffness of the test machine can affect concrete crib 
performance. 

Crack propagation in concrete specimens is dependent 
upon available (potential) energy. Generally, the more 



Sustained loadings of greater than 70 pet of the ultimate 
compressive strength have been shown to reduce the 
strength of concrete specimens. Full testing of concrete 
cribs has not been attempted, but it is assumed a similar 
relationship exists. 



CONCLUSIONS 



Forty-six recommendations or practical considerations 
have been presented to assist in the selection and utiliza- 
tion of longwall face and gate road supports. Many of 
these are a direct outgrowth of the Bureau's ground con- 
trol research in support mechanics, and many offer innova- 
tive solutions to everyday problems faced by mining per- 
sonnel in the control of ground in longwall mining. 
Several of these considerations have application outside of 
longwall mining as well. 

In all of these considerations, one basic theme is em- 
phasized; roof supports that are most compatible with the 
conditions in which they are to be employed should be 
selected. Pursuant to this theme are some fundamental 
concepts that are extremely important and often over- 
looked or unrecognized by mining personnel. These are 
highlighted as follows and are the foundation of the prac- 
tical considerations presented. 

o The control of ground in any mining operation involves 
a rather complex interaction between support elements 
and the strata environment. Both the environment and 
the support behavior and their interaction must be con- 
sidered in any analysis of ground control. 

o A "bigger, the better" attitude in support selection is 
not a good ground control practice. Too much support 
resistance can be just as detrimental and catastrophic as 
too little support. 



o Observed support loading is not a supreme indication 
of required support capacity. Passive support elements 
(shields and crib supports) react loads in response to 
displacements imposed by the strata in proportion the 
stiffness and mechanical properties of the support 
structure. Simply because a support structure fails or 
yields, it does not necessarily mean that larger support 
capacity is required. A smaller capacity support with 
less stiffness or setting force may perform well where 
larger capacity supports have failed. 

o Initial conditions are an important consideration in 
analysis of support behavior and subsequent ground 
control. As much consideration should be given to 
shield setting pressures as yield loads. Setting pressures 
should not simply be set at the maximum available 
pump pressure. 

o Boundary conditions are also important considerations 
in support analysis. Not only is contact configuration 
important when evaluating longwall roof support struc- 
tures, but so are the direction of applied displacement 
(vertical or horizontal) and constrainment to these dis- 
placements. Lack of consideration of these parameters 
can lead to erroneous evaluations of the integrity of the 
support. 



22 



BIBLIOGRAPHY 



Barczak, T. M. Impact of Horizontal Load on Shield Supports. 
Paper in Proceedings of the Fourth Conference on Ground Control in 
Mining, ed. by S. S. Peng and J. H. Kelly. WV Univ., 1985, pp. 50-57. 

. Optimization of Longwall Supports. Pres. at Soc. Min. Eng. 

AIME Fall 1987 Meeting, Pittsburgh, PA, Oct. 12-13, 1987, 25 pp.; 
available upon request from T. M. Barczak, BuMines, Pittsburgh, PA. 

. An Overview of the Bureau's Mine Roof Support Studies 

Research Program. Pres. at 1985 AMC Coal Convention, Pittsburgh, 
PA, May 12-15, 1985; 13 pp.; available upon request from T. M. 
Barczak, BuMines, Pittsburgh, PA. 

. Resultant Load Vector Studies on Shield Supports. Paper 

in Proceedings of the 21st International Conference of Safety in Mines 
Research Institute, Sydney, Australia, Oct. 15-21, 1985, 5 pp. 

. Rigid-Body and Elastic Solutions to Shield Mechanics. 

BuMines RI 9144, 1987, 20 pp. 

. State-of-the-Art Testing and Analysis of Mine Roof Support 

Systems. Paper in Eastern Coal Mine Geomechanics. Proceedings: 
Bureau of Mines Technology Transfer Seminar, Pittsburgh, PA, 
November 19, 1986. BuMines IC 9137, 1987, pp. 3-11. 

Barczak, T. M., and W. S. Burton. Assessment of Longwall Roof 
Behavior and Support Loading by Linear Elastic Modeling of the 
Support Structure. BuMines RI 9081, 1987, 7 pp. 

. The Significance of Specimen Stiffness and Post Yield 

Characteristics on Passive Roof Support Design. Paper in Proceedings 
of the Sixth International Conference on Ground Control in Mining, 
Morgantown, WV, June 9-11, 1987. WV Univ., 1987, pp. 219-226. 

. Three-Dimensional Shield Mechanics. BuMines RI 9091, 

1987, 23 pp. 

Barczak, T. M., and R C. Garson. Shield Mechanics and Resultant 
Load Vector Studies. BuMines RI 9027, 1986, 43 pp. 



Barczak, T. M., and R C. Garson. A Technique to Measure Resul- 
tant Load Vector on Shield Supports. Ch. 68 in Rock Mechanics in 
Productivity, Protection (25th Symp. on Rock Mechanics, Chicago, IL, 
June 25-27, 1984). Soc. Min. Eng. AIME, 1984, pp. 667-679.2 

Barczak, T. M., and C. A. Goode. Considerations in the Design of 
Longwall Mining Systems. Published in Proceedings for State-of-the- 
Art Ground Control in Longwall Mining and Mine Subsidence 
(Honolulu, HI, Sept. 4-5, 1982). Soc. Min. Eng. AIME, 1982, pp. 39- 
50. 

Barczak, T. M., and S. J. Kravits. Shield-Loading Studies at an 
Eastern Appalachian Minesite. BuMines RI 9098, 1987, 81 pp. 

Barczak, T. M., and D. E. Schwemmer. Critical-Load Studies of a 
Shield Support. BuMines RI 9141, 1987, 15 pp. 

. Effect of Load Rate on Wood Crib Behavior. BuMines 

RI 9161, 1988, 11 pp. 

. Stiffness Characteristics of Longwall Shields. BuMines 

RI 9154, 1988, 14 pp. 

Barczak, T. M., and C. L. Tasillo. Factors Affecting Strength and 
Stability of Wood Cribbing: Height, Configuration, and Horizontal 
Displacement. BuMines RI 9168, 1988, 23 pp. 

Barczak, T. M., and P. M. Yavorsky. State-of-the-Art Testing of 
Powered Roof Support Systems. Paper in Proceedings, Second 
Conference on Ground Control in Mining, ed. by S. S. Peng. WV 
Univ., 1982, 64-77 pp. 

Goode, C. A., T. M. Barczak, and J. Jaspal. Support Selection for 
the Multilift Mining Method. Paper in Proceedings, First Annual 
Conference on Ground Control in Mining, ed. by S. S. Peng. WV 
Univ., 1981, pp. 186-200. 



U.S. GOVERNMENT PRINTING OFFICE: 611-012/00,070 



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